models are accurate, compact, and easily portable and stand up well 

 under handling and rugged field conditions. Plates, optical units, and 

 other delicate parts are enclosed to protect them from rough handling, 

 ■water, and dust. 



a. T-2 theodolite 



Because of its good accuracy and ease of handling, the T-2 

 theodolite has become the one most frequently used. It is good for 

 ordinary survey work of third-order accuracy. With the exercise of 

 care and the use of proper daylight conditions and signals or lights 

 at night, second-order accuracy is possible. Angles can be read to 

 one second and estimated to 0.5 second. An astrolabe, or polar attach- 

 ment, makes it possible to do astronomical work with fair accuracy. 

 Azimuths can be determined to within one second of arc. An eyepiece 

 for centering the TV 2 above the station mark is provided. Lighting 

 attachments and battery boxes make the instrument usable for night 

 observations. The lenses have been improved from time to time to 

 give clearer images and improved visibility over long lines, but the 

 length of the line generally is limited to about 10 to 12 nautical miles. 

 The weight of the theodolite is 12 pounds, and that of the container 

 5 1/2 pounds. The cost of the T-2 alone, without tripod or accessories, 

 is approximately $1,400. 



b. T-3 theodolite 



This is a larger model of the T-2 theodolite with which 

 better accuracy is obtainable, but at the expense of portability and 

 ease of handling. The T-3 weighs about 24 pounds and is not as easily 

 transported in the field as the T-2. However, it still is considered 

 portable and rugged enough to be a good field instrument. Where 

 greater accuracy is needed, the T-3 usually is chosen. It is used for 

 primary triangulation or basic control for geodetic work. The circle 

 can be read to 0.2 second. Triangle closures of less than one second 

 are common. With the proper accessories such as the eyepiece prisms 

 and astrolabe, good astronomical work can be done. Three eyepieces 

 of different magnification are supplied with the instrument, and each is 

 used depending on the weather. In heavy haze the 24-power eyepiece 

 is used; in medium haze, the 30-power eyepiece; and with little or no 

 haze, and for astronomical work, the 40-power eyepice is used. Lines 

 25 to 30 nautical miles in length can be observed without difficulty. 

 The T-3 can be purchased for about $2,500; accessories cost an addi- 

 tional $500. 



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